UnionFS

UnionFS is a filesystem service for Linux, FreeBSD and NetBSD which implements a union mount for other file systems. It allows files and directories of separate file systems, known as branches, to be transparently overlaid, forming a single coherent file system. Contents of directories which have the same path within the merged branches will be seen together in a single merged directory, within the new, virtual filesystem.

When mounting branches, the priority of one branch over the other is specified. So when both branches contain a file with the same name, one gets priority over the other.

The different branches may be both read-only and read-write file systems, so that writes to the virtual, merged copy are directed to a specific real file system. This allows a file system to appear as writeable, but without actually allowing writes to change the file system, also known as copy-on-write. This may be desirable when the media is physically read-only, such as in the case of Live CDs.

Uses

In Knoppix, a union between the file system on the CD-ROM or DVD and a file system contained in an image file called knoppix.img (knoppix-data.img for Knoppix 7) on a writable drive (such as a USB memory stick) can be made, where the writable drive has priority over the read-only filesystem. This allows the user to change any of the files on the system, with the new file stored in the image and transparently used instead of the one on the CD.^[1]

UnionFS can also be used to create a single common template for a number of file systems, or for security reasons. It is sometimes used as an ad hoc snapshotting system.

Other implementations

Unionfs for Linux has two versions. Version 1.x is a standalone one that can be built as a module. Version 2.x is a newer, redesigned, and reimplemented one. Version 2.x is the smallest implementation of unioning for Linux, it is heavily tested and inspected by many kernel developers, and it is more efficient.^{[citation needed]}

aufs is an alternative version of unionfs.^[2]

UnionFsFuse is an independent project, implemented as an user space filesystem program, instead of a kernel module or patch. Like UnionFS, it supports copy-on-write and read-only or read-write branches.^{[citation needed]}

Plan 9 from Bell Labs operating system uses union mounts extensively to build custom namespaces per user/processes. The same concept, implemented as "union mount", has also been available in BSD, since at least 1995.^[3]

The GNU Hurd has UnionFS.^[4] As of January 2008, it works, but results in a read-only mountpoint.

mhddfs works like UnionFS but permits balancing files over drives with the most free space available. It is implemented as a user space filesystem.

Sun Microsystems introduced the first implementation of a stacked, layered file system with copy-on-write, whiteouts (hiding files in lower layers from higher layers), etc. as the Translucent File Service in SunOS 3, circa 1986.^[5]

JailbreakMe 3.0, a tool for jailbreaking iOS devices released in July 2011, uses unionfs techniques to speed up the installation process of the operating system modification.^[6]

References

• C. P. Wright et al. (2004). "Versatility and Unix Semantics in a Fan-Out Unification File System (pdf)." (Discusses UnionFS implementations and semantics.)

External links

• Official Linux Unionfs home page
• Bugzilla bug database for Linux Unionfs
• UnionFsFuse FUSE-based alternative implementation of UnionFS
• AuFS is an alternative unionfs for linux.
• FunionFS (Another advanced FUSE based implementation of UnionFS with log, control...)
• Kernel Korner - Unionfs: Bringing Filesystems Together - article from LinuxJournal
• The new implementation of union fs for FreeBSD
• On implementation of a union fs
• LUFS-based unionfs
• Linux Implementations - excellent write-up on history of union file systems for Linux
• The Translucent File Service (white paper), David Hendricks, Sun Microsystems (gzip PostScript)
• mhddfs
• mini_fo: overlay filesystem (rw) for read-only rootfs
• [http://www.freebsd.org/cgi/man.cgi?query=mount_unionfs&sektion=8&apropos=0&manpath=FreeBSD+9.0-RELEASE+and+Ports: FreeBSD mount_union man page]